The invention relates to a method for the production of an open form composed of a plurality of layers which are each two-dimensionally structured. The form is made of a material which can be etched depending on its doping.
The invention relates, in particular, to the production of an open form which is suitable as a xe2x80x9cphotonic crystalxe2x80x9d.
In the context of this description, an open form is understood to mean a form having a non-convex geometry. An open form may have, in particular, a geometry that is not simply connected in the sense of the conventional topology, that is to say be provided with a hole, a channel bounded in two dimensions, a plurality of holes or the like.
Such an open form may, under certain circumstances, function as a photonic crystal, i.e. as a more or less periodic, as it were crystalline structure which, with regard to the transmission of photons (light) behaves like a crystalline semiconductor, in the conventional sense, with regard to the transmission of electrons. A photonic crystal may, under certain circumstances, have a xe2x80x9cphotonic band gapxe2x80x9d by analogy with a semiconductor having an xe2x80x9celectronic band gapxe2x80x9d; this means that the photonic crystal is impervious to a photon having an energy within the photonic band gap. This means that the photonic crystal acts as an essentially perfect mirror for such a photon, if the latter falls onto the crystal from outside. It is this property that explains the interest in photonic crystals for bounding optical waveguides or optical cavity resonators, because unlike a conventionally used means for bounding an optical waveguide or cavity resonator by means of a totally reflecting arrangement, the reflecting property of a photonic crystal is independent of an angle at which a photon to be reflected impinges on the photonic crystal.
An open form designed as a photonic crystal and a method for its production are described in the PCT publication WO 97/04340 A1. The open form is composed of silicon as the material and is produced by means of electrochemical etching. WO 97/04340 A1 also mentions gallium arsenide and aluminum gallium arsenide as suitable materials; the desired open structure can be produced in such a material by reactive ion etching.
A broad overview of the technical field of photonic crystals is provided by the paper xe2x80x9cPhotonic Band Structurexe2x80x9d by E. Yablonovitch, contained in the book edited by C. M. Soukoulis xe2x80x9cPhotonic Band Gaps and Localizationxe2x80x9d, Plenum Press, New York, N.Y., 1993, pages 207 et seq. One example of a photonic crystal is found on page 222 of that paper, FIG. 15. The figure also contains information regarding the production of that photonic crystal.
Also of interest in this context is a paper by R. D. Meade et al., xe2x80x9cNovel Applications of Photonic Band Gap-Materials: Low-loss Bends and High Q Cavitiesxe2x80x9d, Journal of Applied Physics 75 (1994) 4753. That paper shows cavity resonators and waveguides which are bounded by photonic crystals.
A photonic crystal conventionally requires an open form that is constructed in a comparatively complex manner. In addition, there is a desire to produce such an open form from a material of the kind used for electronic semiconductor components, and thus to enable integration of the technology of photonic crystals with the technology of semiconductor optoelectronics.
It is accordingly an object of the invention to provide a method of producing an open form, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which can be composed of a plurality of in each case two-dimensionally structured layers and is composed of a material which can be etched in dependence of its doping.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of producing an open form composed of a plurality of layers each structured with a two-dimensional pattern, the open form being made of silicon that can be etched depending on a doping. The method comprises the following steps:
providing a first layer of silicon, and marking a portion of the first layer belonging to a form to be produced, by doping at least one zone of the first layer, and leaving unmarked portions;
applying a further layer of silicon, and marking a portion of the further layer belonging to the form to be produced, by doping at least one zone of the further layer, and leaving unmarked portions, each doping process producing p-type conductivity with a doping atom concentration above 1015 per cm3 in the silicon; and
removing all unmarked portions by etching with an alkaline etching liquid, containing ethylene diamine and pyrocatechol, depending on a respective doping of each layer.
The open form to be produced is structured in such a way that it can be composed of in each case two-dimensionally structured layers. Structuring of a layer is deemed to be two-dimensional when it is homogeneous along a thickness direction of the layer; thus, the structure is an essentially planar structure. By corresponding doping of each layer, each layer portion belonging to the form is marked and thereby prepared for the etching process which releases the open form to be produced from the complex of layers which is homogeneous except for the doping. The dopings can either facilitate the etching of the material (the open form is then produced from the material left undoped) or make it more difficult (the open form is then produced from the doped material). There are, moreover, no fundamental limitations with regard to the application of the at least one further layer to the first layer. A further layer may either be grown, that is to say produced directly on the substrate already present by phase transformation, or be produced as an initially separate solid, if appropriate already doped as desired, and be connected to the substrate already present by subsequent xe2x80x9cwafer bondingxe2x80x9d. A wide variety of etching methods which allow selective etching of a material depending on a doping are well known in the field of the technology of electronic semiconductors. In the instant case it is silicon that is to be etched and the etching can be adapted to the respective doping.
In accordance with an added feature of the invention, the further layer is formed with a growth process. In this case, furthermore, it is preferable for the first layer to be provided as a single crystal and each further layer to be grown epitaxially, that is to say with continuation of the monocrystalline structure.
In accordance with an additional feature of the invention, before each doping process, the corresponding layer is covered with a mask outside each zone and the mask is removed directly after the doping process.
As noted above, the material of the open form is silicon. The doping process can be effected with the production of p-type conductivity in the silicon, particularly preferably by the introduction of boron. It is additionally preferred, in the case of each doping process, to produce a concentration of doping atoms, thus in particular boron, above 1015 per cm3, preferably above 1020 per cm3, in the material. The etching can then be effected by an alkaline etching liquid; an exemplary embodiment in this regard will be explained. Marking the portions belonging to the open form by p-conductive doping has the advantage that the p-conductively doped silicon is usually dissolved by means of an alkaline etching liquid to a significantly lesser extent than undoped silicon.
In the method, an open form is preferably produced with a structure which is periodically repeated at most four times. This is of particular importance when the open form to be produced is a-photonic crystal, because the functioning of a photonic crystal can be detrimentally affected if the production-dictated dimensional tolerances of a structure that is repeated too often become apparent to an excessively great extent.
A preferred development of the method provides for each doping process to be effected by means of a masking method which is well known in the technology of electronic semiconductor components. In this case, before each doping process, a mask covering the corresponding layer outside each zone to be doped is applied, then the desired doping is performed and the mask is removed again directly after the doping process.
As noted above, the form to be produced is preferably a photonic crystal.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for the production of an open form, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.